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LT8708
80V Synchronous 4-Switch Buck-Boost DC/DC Controller with Bidirectional Capability
FEATURES
n Single Inductor Allows VIN Above, Below, or Equal to VOUT
n Six Independent Forms of Regulation n VIN Current (Forward and Reverse) n VOUT Current (Forward and Reverse) n VIN and VOUT Voltage
n Forward and Reverse Discontinuous Conduction Mode Supported
n Supports MODE and DIR Pin Changes While Switching n VINCHIP Range 2.8V (Need EXTVCC > 6.4V) to 80V n VOUT Range: 1.3V to 80V n Synchronous Rectification: Up to 99% Efficiency n Available in 40-Lead (5mm × 8mm) QFN with High
Voltage Pin Spacing and 64-Lead (10mm × 10mm) eLQFP n AEC-Q100 Qualified for Automotive Applications
APPLICATIONS
n High Voltage Buck-Boost Converters n Bidirectional Charging System n Automotive 48V Systems
DESCRIPTION
The LT®8708 is a high performance buck-boost switching regulator controller that operates from an input voltage that can be above, below or equal to the output voltage. Features are included to simplify bidirectional power conversion in battery/capacitor backup systems and other applications that may need regulation of VOUT, VIN, IOUT, and/or IIN. Forward and reverse current can be monitored and limited for the input and output sides of the converter. All four current limits (forward input, reverse input, forward output and reverse output) can be set independently using four resistors on the PCB.
The MODE pin can select between discontinuous conduction mode (DCM), continuous conduction mode (CCM), hybrid conduction mode (HCM) and Burst Mode® operation. In combination with the DIR (direction) pin, the chip can be configured to process power only from VIN to VOUT or only from VOUT to VIN. With a wide 2.8V to 80V input and 1.3V to 80V output range, the LT8708 is compatible with most solar, automotive, telecom and battery-powered systems.
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TYPICAL APPLICATION
10V TO 16V BATTERY
VBAT1
+ +
–
POWER TRANSFER DECISION LOGIC
RVS (0V) FWD (3V)
LD033
12V Bidirectional Dual Battery System with FHCM and RHCM
ILIM
TO DIODE DB1
ILIM
TO DIODE DB2
VBAT2
+ + 10V
TO 16V
– BATTERY 100
TG1 BOOST1 SW1 BG1 CSP CSN CSNIN
CSPIN VINCHIP SHDN FBIN VINHIMON
DIR
SWEN LDO33
RVSOFF MODE VC
LT8708 RT
GND BG2 SW2 BOOST2 TG2
CSPOUT
CSNOUT EXTVCC VOUTLOMON FBOUT INTVCC
GATEVCC IMON_OP IMON_ON
ICN ICP IMON_INP IMON_INN SS SYNC CLKOUT
8708 TA01a
126kHz
DB1 DB2
TO TO BOOST1 BOOST2
EFFICIENCY (%)
99 98 97 96 95 94
10
Efficiency
VBAT2 = 13.5V IOUT = 15A
12
14
VBAT1 (V)
16
8708 TA01b
Rev. C
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1
LT8708
TABLE OF CONTENTS
Features...................................................... 1
Applications................................................. 1
Typical Application ......................................... 1
Description.................................................. 1
Absolute Maximum Ratings............................... 3
Pin Configuration........................................... 3
Order Information........................................... 4
Electrical Characteristics.................................. 4
Typical Performance Characteristics.................... 9
Pin Functions............................................... 13
Block Diagram.............................................. 15
Operation................................................... 16
Typographical Conventions...................................... 16
Start-Up................................................................... 16
Start-Up: SHDN Pin............................................. 16
Start-Up: SWEN Pin............................................. 16
Start-Up: Soft-Start of Switching Regulator........ 17
Control Overview..................................................... 17
Power Switch Control ............................................. 18
Switch Control: Buck Region (VIN >> VOUT)........ 19
Switch Control: Buck-Boost (VIN ≅ VOUT)........... 19
Switch Control: Boost Region (VIN << VOUT)....... 20 Switch Control: Soft-Start...................................20
Uni and Bidirectional Conduction.............................20
Bidirectional Conduction: CCM ........................... 21
Unidirectional Conduction: DCM.......................... 21
Unidirectional Conduction: HCM..........................22
Unidirectional Conduction: Burst Mode...............22
Error Amplifiers.......................................................22
VOUT Regulation and Sensing..................................23 VOUT: Regulation..................................................23 VOUT: Above Regulation.......................................23 VOUT: Below Regulation and Undervoltage..........23
VIN Regulation and Sensing..................................... 24 VIN: Regulation.................................................... 24 VIN: Above Regulation and Overvoltage............... 24 VIN: Below Regulation......................